The invention relates to a plasticizing and injection device for a plastics processing machine.
Injection molding machines are known having a so-called reciprocating screw for jointly execute the function plasticizing and injection. The screw is caused to rotate in order to melt and transport the plastic. Furthermore, the screw is movable axially for injecting the plastic melt into a mold and has a non-return valve at its leading end. Such a plasticizing and injection device is characterized by its simplicity and its versatility with respect to different applications. A drawback of such a plasticizing and injection device is foremost the adverse effect on the homogeneity of the melt as a result of the rearward movement of the screw during plasticizing because of the absence of a constant screw length for plasticizing and homogenizing the plastic pellets within a cycle. Moreover, the screw is idle during injection and holding pressure phase, thereby adversely affecting the cycle time. Further disadvantageous is the balance that must be struck between plasticizing capacity, on one hand, and injection pressure, on the other hand, for dimensioning of the screw.
To address the afore-mentioned drawbacks, proposals have been made for so-called injection molding compounders to provide a continuously operating twin-screw extruder and downstream thereof one or more injection plunger and cylinder units (DE 19828770 A1, WO 86/06321 A1).
German publication DE 1105153 further discloses an injection molding machine which also provides a functional split between plasticizing, on one hand, and injection, on the other hand. This document discloses a plasticizing and injection device having a plasticizing cylinder which accommodates a rotatable plasticizing screw, a melt chamber anteriorly of the plasticizing screw for receiving and temporary storage of plastic melt, and an injection cylinder downstream of the melt chamber. The injection cylinder can be moved through displacement of the entire clamping unit against the injection cylinder and then moved as unit against a stationary injection plunger to transfer plastic melt into the closed injection mold. The melt chamber is defined on the opposite side of the injection plunger by a recess in the injection plunger, on one hand, and by an extension piece on the plasticizing cylinder, on the other hand. The plasticizing cylinder can be moved back and forth in relation to the stationary injection plunger by means of plunger and cylinder units in order to be able to fill the melt chamber, on one hand, and to displace plastic melt from the melt chamber into the injection cylinder, on the other hand. A channel extends in the stationary injection plunger for connecting the melt chamber with the injection cylinder and is blocked by a ball check valve during injection to prevent backflow of melt. A drawback is, on one hand, the comparably great diameter of the injection plunger, which adversely affects a precise metering of the shot volume. Heating of the melt chamber and injection cylinder has also being shown relatively difficult because the chambers, bounded by the stationary injection plunger, are temporarily exposed and temporarily plunge into a housing. Also the overall construction appears complex and cumbersome as the entire clamping unit has to be moved for operating the injection cylinder.
DE 4401026 discloses a further plasticizing and injection device to provide a functional split between plasticizing, on one hand, and injection, on the other hand. This device includes a plasticizing cylinder with a rotating plasticizing screw and an injection plunger axially movable in the plasticizing screw. The injection plunger is realized in the form of a piston rod with a non-return valve. Provided inside the plasticizing screw is an accumulator chamber for plastic melt which chamber is subdivided by the non-return valve into two separate accumulator chambers of a volume which is dependent on the position of the non-return valve. Both accumulator chambers communicate with the plasticizing space via melt channels in the plasticizing screw. Plastic melt flows via these channels during the plasticizing process to the front and/or rear accumulator chamber in dependence on the position of the injection plunger. After both accumulator chambers have been filled, the injection plunger moves backwards and plastic melt is displaced from the rear accumulator chamber through the open non-return valve to the front accumulator chamber. The injection plunger is moved forwards to inject into an injection mold, without interrupting the rotation of the plasticizing screw. A drawback is hereby the required high energy consumption for heating the accumulator chamber inside the plasticizing screw since heat has to penetrate through the entire thickness of the plasticizing screw. Moreover, an uncontrolled backflow of plastic melt through the melt channels into the screw flights during the injection process. As a result, it becomes difficult to precisely and reproducibly realize the shot volume. Furthermore, problems may be encountered when closing the non-return valve because, as a result of the backflow of plastic melt into the screw flights, the pressure buildup may be inadequate upstream of the non-return valve to enable a complete closing of the non-return valve.
GB-A-1015092 discloses a plasticizing and injection device which also describes a functional split between plasticizing, on one hand, and injection, on the other hand. A rotatable plasticizing screw is arranged in a plasticizing cylinder in an axially immovable manner and an injection cylinder is provided upstream of the plasticizing cylinder. An injection plunger made of two separate parts is situated inside the plasticizing screw, with the rear part extending out of the screw and movable linearly in injection direction by a single-action piston and cylinder assembly. The front part projects forwardly beyond the screw and includes a headpiece with channels in which ball check vales are disposed. Both parts impact one another inside the screw. At metering, plastic melt is advanced forwardly through a very narrow gap between the injection plunger and the injection cylinder and through channels extending in the head of the injection plunger to the space anteriorly of the injection plunger. As pressure builds up, the forward part of the injection plunger is moved backwards and thus also the abutting rear part of the injection plunger. As soon as the required amount of plastic melt is realized and the injection plunger assumes its initial position for the injection process, the rotary drive of the screw is switch off and melt production is stopped. This is necessary insofar as no melt chamber is available which could accept and temporarily store plastic melt during the injection process and possibly also during the holding pressure phase. If the plasticizing screw were to continue to operate, significant pressure would buildup behind the ball check valves and generate a force upon the injection plunger in opposition to the injection direction. This known plasticizing and injection device is thus hardly suitable for short cycle times. In addition, there is a significant risk of unwanted heating of the plastic melt in the very narrow gap between injection plunger and injection cylinder, possibly causing decomposition. As the injection plunger can be operated only in injection direction, there is no option to effect a momentary pressure relief in the melt immediately before injection through a slight backward stroke, a fact that is oftentimes considered beneficial.
German publication DE 1084474 discloses a plasticizing and injection device of a type involved here for a plastics processing machine, having a rotatable plasticizing screw, disposed axially immovable in a plasticizing cylinder, and an injection plunger, arranged in the plasticizing screw and axially movable back and forth by means of a linear drive, with the injection plunger having a non-return valve and with an injection cylinder being provided upstream of the plasticizing cylinder. The plasticizing screw does not completely fill the plasticizing cylinder; rather the forward end of the plasticizing screw is situated offset to the rear in relation to the forward end of the plasticizing cylinder in order to form an antechamber for accepting and temporarily store plastic melt. This antechamber is sized large enough to accept at a distance to its walls the entire injection plunger, which has a diameter greater than the piston rod and is provided with the non-return valve, in the initial position for executing the injection process. As the injection plunger moves out of the injection cylinder during each injection process, implementation of a precise metering becomes difficult. Moreover, the transition from the antechamber to the injection cylinder can easily be damaged when the injection plunger slightly deviates from the injection axis for example. In addition, there are limitations as far as selection of useable non-return valves is concerned because the locking ring may not move out of the injection cylinder.